Try this experiment. Take two black ABS plastic fittings (not PVC or CPVC) and try to glue the OUTSIDE surfaces together. ABS is usually used for drain pipes, PVC for electrical, and CPVC for water. Two couplings will demonstrate the problem rather nicely. Because concentric pipe connections have a rather large common surface area, the glue does not need to be very strong, gap filling, or tolerant of minor deflections. The result is that such a small surface external glue connection will easily crack, flake, or crumble. If the two cylindrical surfaces are not perfectly straight and parallel, the glue filled gap will certainly crack. (Note that ABS does not require a primer).
Good idea. I had that idea once and used a grinding wheel to produce PVC (not ABS) powder. I ended up melting the PVC and had to resurface the grinding wheel. I suppose a slower and rougher grinding wheel would have done a better job. These daze, if I need filler, I use various powdered metals "borrowed" from the local powdered metal fabrication shop.
Yep. That's roughly what my little experiment demonstrates. There's more to making a strong glue joint than just the glue. Most of the plastic glues are acrylic or some other plastic dissolved in a solvent (usually heptane). When the solvent evaporates, the acrylic remains behind to make the connection. If there's no adhesion between the acrylic and the ABS, it's not going to work. However, if the solvent softens the surface of the ABS slightly, the acrylic mixes with the ABS along the glue line, and the joint is much stronger. The plastic epoxy that I recommended seems to use these methods, with the added bonus of a glue that's stronger than acrylic. Try gluing the same two ABS fitting together with this stuff and note the difference in strength.
I beg to differ. Hot glue is rather brittle, does not stick to smooth surfaces, and will fall apart on a hot day. There's plenty of places where I will use the stuff, but not for anything I want to be permanent.
One trick I use for plastic repair is plastic welding. It uses exactly the same techniques as oxy-acetylene gas welding, except that can be done with a cheap propane torch or hot air source. Of course, it works at much lower temperatures. I melt the plastic with the torch flame and add filler in the form of a plastic rod. I use my SMD desoldering station (Saike 852D+) with a very small orifice tip. Keep the air flow low, so that the hot air doesn't blow the plastic away from the weld area. Google for "hot air plastic welding". Lots of other products and YouTube videos.
However, hot air plastic welding does take some practice. One of my early mistakes was trying to weld the plastic mounting studs back onto the plastic keyboard bezel on a Toshiba laptop. I used too much heat resulting in a very visible melt zone on the front panel. To prevent a repetition of this mistake, I now use a wet sponge on the opposite side of the weld area. When I hear the water sizzle, I stop.